This invention relates to an equalizer incorporated in a data receiver apparatus used in, for example, a digital mobile radio telephone system.
An equalizer of this kind is described in IEEE Global Telecommunications Conference & Exhibition, Dallas, Tex., Nov. 27-30, 1989, Conference Record Vol. 1 of 3, pp. 95-101. Such a will be first described before describing the present invention in detail later.
FIG. 1 shows the structure of one form of a burst signal processed by an equalizer incorporated in a data receiver apparatus used in, for example, a digital mobile radio telephone system, and FIG. 2 shows the waveform of an impulse response of a transmission channel. This impulse response includes not only that of an impulse signal transmitted from a transmitting antenna and directly received by a receiving antenna but also that of the impulse signal reflected by, for example, a building and then received by the receiving antenna with a delay time. FIG. 3 shows the structure of the prior art equalizer incorporated hitherto in the data receiver apparatus. Practically, two equalizers, each of which is as shown in FIG. 3, are used in the data receiver apparatus. In FIG. 3, a received signal is applied from an input terminal 1 to a plurality of delay elements 2. Outputs from a plurality of weighting elements 3 having respectively different weight coefficients are applied to an adder 4, and the output from the adder 4 is applied to a comparator 5 to appear as an output 6 from the equalizer. At the same time, the output 6 from the equalizer is applied to a plurality of delay elements 9 associated with a plurality of weighting elements 10.
The operation of the prior art equalizer shown in FIG. 3 will now be described. Referring to FIG. 3, the received signal is stored in a delay line of each of the delay elements 2 (six samples in the arrangement shown in FIG. 3), and, after the outputs appearing at individual taps are multiplied by the weight coefficients of the weighting elements 3 respectively, the outputs from the weighting elements 3 are added together by the adder 4. Thus, a digital filter of FIR (finite impulse response) type provides an output in which waveform distortion due to the signal transmission through the transmission channel is compensated. The comparator 5 acts to convert the output from the FIR type digital filter into the corresponding amplitude. (For example, in the case of GMSK modulation, the comparator 5 generates its output +1 when its input is positive and -1 when its input is negative.) This output from the comparator 5 provides the output 6 of the equalizer and is stored in a delay line of the each of the delay elements 9. After the outputs appearing at individual taps are multiplied by the weight coefficients of the weighting elements 10 respectively, the outputs from the weighting elements 10 are added together by the adder 4. Thus, a digital filter of IIR (infinite impulse response) type provides an output in which waveform distortion due to the signal transmission through the transmission channel is compensated. In FIG. 3, the digital filter of the FIR type and that of the IIR type are indicated by the blocks 11 and 12 surrounded by broken lines respectively.
The IIR type digital filter referred to above is effective only for waveform distortion due to waveform components (23 to 26 in FIG. 2) appearing after a main waveform component 22 (the component having the highest power level) relative to time. On the other hand, the FIR type digital filter is effective for both the components appearing after and before the main waveform component relative to time. However, the IIR type digital filter is more effective than the FIR type digital filter for the components appearing after the main waveform component relative to time.
Suppose now the case where the equalizer having the structure shown in FIG. 3 is used to deal with a burst signal in which a reference signal part is interposed between a former half data part and a latter half data part as shown in FIG. 1. In the burst signal shown in FIG. 1, both the former and latter half data are voice data subjected already to error correction coding, and the reference signal is in the form of a fixed pattern determined to meet the system. The reference signal part used in the burst signal is a digital pattern of "1" or "0". In the equalizer, the initial values of the weight coefficients of the weighting elements are determined on the basis of the reference signal. Thus, the latter half data part is equalized in a direction as shown by the arrow B in FIG. 1, while the former half data part is equalized in a direction as shown by the arrow A which is opposite to the direction B relative to time t. Therefore, in such a case, it is necessary to use two equalizers each having the structure shown in FIG. 3. Further, in FIG. 3, the range of signal waveform delays is selected to be, for example, 5T, where T represents the length of time of one symbol and is the reciprocal of the bit rate in the case of a binary modulation, such as, the GMSK modulation. That is, T=5 .mu.s when the bit rate=200 kb/s. When the value of T is so selected, the maximum number of taps required for the FIR type digital filter is 6, because this digital filter is effective for waveform distortion due to both the waveform components appearing after and before the main waveform component relative to time. On the other hand, the maximum number of taps required for the IIR type digital filter is 5, because this digital filter is effective only for waveform distortion due to the components appearing after the main waveform component relative to time. Thus, taking into consideration the condition of the signal transmission channel, it is necessary to provide the maximum number of taps for each of these digital filters. Therefore, in this case, each of the two equalizers includes the 6-tap FIR type digital filter and the 5-tap IIR type digital filter.
As described above, the FIR type digital filter and the IIR type digital filter are combined to form the prior art equalizer incorporated in the data receiver apparatus. Therefore, the equalizer can compensate both the waveform distortion due to the components appearing after the main waveform component relative to time and that due to the components appearing before the main waveform component relative to time.
However, the prior art equalizer incorporated in the data receiver apparatus has had such various problems that, because the equalizer requires a large number of taps, the number of signal processing operations is correspondingly increased, and difficulty is encountered for reducing the power consumption and size of the equalizer.